Semiconductor lasers used for an optical communication system, an optical information processing system, etc. have been required to have much higher performance than ever. At the same time, optical devices having high yield have been required to be fabricated by using large area wafers, so that the optical devices thus fabricated will be reduced in cost to be applied to subscriber's optical communication systems. In order to meet these requirements, it is required that crystal growth is carried cut by vapor phase epitaxy such as metal organic vapor phase epitaxy making it possible to grow crystal on a large area with high uniformity. Further, if vapor phase epitaxy is used, the fabrication of quantum well-semiconductor laser having several features such as low threshold value, high efficiency-operation, etc. is possible to be carried out.
A conventional method for fabricating an optical semiconductor laser by using MOVPE is described on pages 226 to 230 in "Journal of Crystal Growth, Vol. 107".
In the conventional method, a distributed feedback type (DFB) laser operating in a single mode is fabricated by using the steps of farming a grating, an n-InGaAsP guide layer, an InGaAsP active layer, and a p-InP cladding layer successively on an n-InP substrate, forming a stripe-shaped SiO.sub.2 film having a width of 2 .mu.m on the p-InP cladding layer, wet-etching the layers grown on the n-InP substrate except for the portion masked by the SiO.sub.2 film, forming a p-InP layer and a p.sup.+ -InGaAs cap layer on the whole surface on the n-InP substrate thus processed to bury the mesa structure, implanting protons into the cap layer to provide high resistance regions on the both sides of the active layer, and providing n- and p-electrodes for injecting current to the active layer.
The DFB laser thus fabricated has a structure for confining a current comprising a buried ridge and high resistance regions surrounding the buried ridge.
In the conventional method for fabricating an optical communication-semiconductor laser, however, there is a disadvantage in that the width of a waveguide is difficult to be well controlled due to the occurrence of side-etching, because the waveguide is fabricated by the wet-etching using the SiO.sub.2 film as a mask, although thickness of the layers are adequately controlled in vapor phase epitaxy such as MOVPE. For instance, even if the width of the SiO.sub.2 film is formed precisely by 2 .mu.m, the width of the active layer is deviated due to the occurrence of the etching on the sides of the active layer and the mesa-structure. In a process using a large diameter wafer such as a two inch-substrate, especially, the deviation ranges large in the wafer. In the mesa-etching, if dry-etching is used, there is a further disadvantage in that the active layer is damaged to result in defects, although the width thereof is well controlled.
Due to the deviation of the waveguide including the active layer and the defects of the active layer, threshold value, oscillation wavelength, beam pattern, etc. are deteriorated to lower the reliability thereof, so that the yield of fabricating optical semiconductor devices is lower, and operation a expected is not obtained.
In the conventional method as described above, there is a still further disadvantage in that a process time becomes long, because at least two steps of crystal growth are required.